Plastic molding device for a semiconductor element

ABSTRACT

A plastic molding device for a semiconductor element for plastic molding a semiconductor element on a lead frame, which comprises: an upper platen, a lower platen, and a movable platen movably provided at a tie bar which is provided between said two platens; an upper metal mold and a lower metal mold provided at the upper platen and the movable platen, respectively; a motor for applying a raising force for driving the movable platen to; a driving force conversion mechanism for converting the rotation force of the motor into a reciprocative force; and a double toggle mechanism for giving the reciprocative force to the movable platen.

This application is a continuation of copending application Ser. No.829,087, filed on Feb. 13, 1986, now U.S. Pat. No. 4,755,124.

FIELD OF THE INVENTION

The present invention relates to a plastic molding device for asemiconductor element for plastic molding a semiconductor element on alead frame.

BACKGROUND OF THE INVENTION

Generally, a plastic molding device for a semiconductor element has anupper platen for holding an upper metal mold, a lower platen as a bases,and a movable platen for holding a lower metal mold. The movable platenis raised up to mold-clamp the upper and the lower metal molds, and alead frame and a semiconductor element placed between the molds areplastic molded.

FIGS. 1 and 1a show a prior art plastic molding device for asemiconductor element. An upper platen is provided for holding an uppermetal mold 8. A tie bar 3 is provided between the upper platen 1 and thelower platen 2. There is also provided a movable platen 4 between theupper platen 1 and the lower platen 2. In order to conduct mold clampingthe movable platen 4 is driven upwardly by a main oil pressure piston 5and auxiliary oil pressure pistons 6, and the lower metal mold 9 fixedon the movable platen 4 is pushed towards the upper metal mold 8. Then,a plastic molding tablet 31 is inserted into the tablet insertionaperture 30 provided in plurality inside the lower metal mold 9, andthere are placed a lead frame and a semiconductor element on the lowermetal mold 9. A power unit 7 including a motor and an oil pressure tankis provided for supplying oil pressure to cylinders each containing themain oil pressure piston 5 and the auxiliary oil pressure piston 6.

After the mold clamping is conducted the press pressure is held at apredetermined pressure, and the plunger 32 provided at the lower metalmold 9 is driven to rise up by an oil pressure so as to inject plasticsinto the metal mold. After the semiconductor element is plastic moldedby the injected plastics, the movable platen 4 is lowered so as toseparate the upper metal mold 8 and the lower metal mold 9, and theplastic molded lead frame is taken out.

In this prior art device the plunger 32 is driven by oil pressure,whereby it is difficult to adjust the speed of the plunger 32 and thepress pressure at high precision, resulting in difficulty in a stableplastic molding, additionally the maintenance of the device becomesquite troublesome because inspection for oil leakage and exchange ofsealing member are required. Furthermore, the device including the powerunit occupies a large space, resulting in an increased cost.

Another prior art plastic molding device for a semiconductor element isdisclosed in Japanese Utility Model Laid-open Publication No. Sho.59-9538. In this device a DC servomotor is used for driving a spindlefor plastics injection, wherein the spindle is provided with a plunger.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a plastic moldingdevice for a semiconductor element capable of making the maintenancethereof easy, adjusting the pressure easily at a high precision, andfurther miniaturizing the whole device and reducing the cost of thedevice.

It is a second object of the present invention to provide a plasticmolding device for a semiconductor element of motor-operated systemcapable of transmitting the driving force of a motor without the use ofgears, and further obtaining a simple construction with no oil leakage.

It is a third object of the present invention to provide a plasticmolding device for a semiconductor element capable of conducting aplastic molding with high accuracy even if the metal molds are curveddue to the pressure.

Other objects and advantages of the present invention will becomeapparent from the detailed description given hereinafter; it should beunderstood, however, that the detailed description and specificembodiment are given by way of illustration only, since various changesand modifications within the spirit and scope of the invention willbecome apparent to those skilled in the art from this detaileddescription.

According to the present invention, there is provided a plastic moldingdevice for a semiconductor element for plastic molding a semiconductorelement on a lead frame, which comprises: an upper platen, a lowerplaten, and a movable platen movably provided at a tie bar which isprovided between said two platens; an upper metal mold and a lower metalmold provided at the upper platen and the movable platen, respectively;a motor for applying a driving force for driving the movable platen torise up; a driving force conversion mechanism for converting therotation force of the motor into a reciprocative force; and a doubletoggle mechanism for giving the reciprocative force to the movableplaten.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

FIG. 1 is a schematic view showing a prior art plastic molding devicefor a semiconductor element:

FIG. 1a is an enlarged cross-sectional view showing a portion includingthe plunger of FIG. 1;

FIG. 2 is a schematic view showing the whole construction of a plasticmolding device for a semiconductor element according to one embodimentof the present invention:

FIG. 3 is an enlarged view of one portion of the device of FIG. 2:

FIG. 4 is an enlarged cross-sectional view showing the plunger portionthereof:

FIG. 5 is a schematic view showing the mold clamping state of thedevice:

FIG. 6 is an enlarged perspective view showing the position detectionmeans of the device:

FIG. 7 is a schematic view showing a construction of the torquetransmission system and the control system of the device:

FIG. 8 is a diagram showing a flow chart for exemplifying the operation:

FIG. 9 is a schematic view showing a construction of a secondembodiment:

FIG. 10 is a schematic view showing a construction of a fourthembodiment:

FIG. 11 is a diagrammatic view showing a construction of a controlapparatus of the device:

FIG. 12 is a schematic view showing a main portion of a fifthembodiment:

FIG. 13 is a schematic view showing a mold clamping state of a prior artdevice:

FIG. 14 is a schematic view showing a construction of a sixthembodiment: and

FIG. 15 is a schematic view showing the mold clamping state of thedevice.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to explain the present invention in detail, reference will beparticularly made to FIGS. 2, 3, and 4.

In the above noted Figures, the reference numeral 10 designates an upperplaten, the numeral 11 designates a lower platen, the numeral 12designates a tie bar provided between the upper platen 10 and the lowerplaten 11. The numeral 13 designates a movable platen movably providedat the tie bar 12. The numeral 14 designates an upper metal mold fixedto the upper platen 10. The numeral 15 designates a lower metal moldfixed to the movable platen 13. A plurality of tablet insertionapertures 23 into which molding plastic (tablet) 22 is inserted areprovided at the lower metal mold 15 as shown in FIG. 4, and a plunger 24is provided in the aperture 23 movably up and downwards, and it isdriven by an external plunger driving apparatus.

The reference numeral 16 designates an AC servomotor for giving adriving force for driving the movable platen 13 upwards. The numeral 17designates a torque transmission system for transmitting the drivingforce of the AC servomotor 16 to the movable platen 13. The numeral 18designates a small bevel gear provided at the output axis of the torquetransmission system 17. The numeral 19 designates a large bevel gearengageable with the small bevel gear 18. The numeral 20 designates aball screw at one end of which the large bevel gear 19 is fixed. Thenumeral 21 designates a nut engageable with the ball screw 20. A drivingforce conversion mechanism for converting the rotation force of the ACservomotor 16 into a reciprocative force is constituted by the ballscrew 20 and the nut 21.

Furthermore, the numeral 30 designates a double toggle mechanismintended to receive the reciprocative force from the driving forceconversion mechanism, to convert this force into a tremendously largeforce, and to give it to the movable platen 13. This double togglemechanism is constituted symmetrically at the left and right viewed fromthe center of the ball screw 20.

The construction of the right side portion shown in the drawing isdescribed. The numeral 31 designates a nut housing as an input link tothe center of which the nut 21 is fixed. The numeral 32 designates anintermediate link whose one end is connected to the nut housing 31rotatably. The numeral 33 designates an L-shaped link having an L-shapedtransverse cross-section whose one end is rotatably connected to thelower platen 11, and whose other end is connected to the other end ofthe intermediate link 32, respectively. The numeral 34 designates anoutput link whose one end is rotatably connected to the L-shaped link33, and whose other end is rotatably connected to the rib 13a providedat the lower surface of the movable platen 13. In this embodiment themold clamping by the upper metal mold 14 and the lower metal mold 15 isconducted in a position that the output link 34 is positionedvertically, that is, at the neighborhood of the position where thedouble toggle mechanism 30 is located at the uppermost dead point.

The reference numeral 40 designates a position detection means providedat the movable platen 13 for detecting the position of the movableplaten 13. As seen in FIG. 6, this position detection means comprises arotary encoder 42 including a rack 41 fixed to the movable platen 13 anda pinion 42a which is rotated by the movement of the rack 41 through agear 44, and this rotary encoder 42 is fixed to a stay 43 attached tothe lower platen 11.

FIG. 7 shows an example of a construction of the torque transmissionsystem 17. In this torque transmission system 17 a first torquetransmission series 171 having a large reduction ratio is constituted bya first input gear G1, a first clutch C1, an input axis 17a, a secondinput gear G2, a second output gear G4, and an output axis 17b, and asecond torque transmission series 172 having a small reduction ratio isconstituted by the first input gear G1, a first output gear G3, a secondclutch C2, the second output gear G4, and an output axis 17b. These twotorque transmission series 171 and 172 are switched by clutches C1 andC2, and the speeds thereof are controlled by a brake 65 at a desiredtime. The numeral 50 designates a control apparatus designed to receivethe output from the position detection means 40 at its input, and tocontrol the AC servomotor 16 and the brake 65, and to switch the torquetransmission system 17.

The device will be operated as follows:

At first the movable platen 13 is returned to the original position ofthe automatic operation. In this state a lead frame and a semiconductordevice are loaded to the lower mold 15 by an inloader, and a tablet 22is inserted into the tablet insertion aperture 23 of the lower mold 15,and thereafter an operation start switch of the device is turned on.Then, the AC servomotor 16 starts to rotate, and the rotation force ofthe motor 16 is transmitted to the ball screw 20 through the drivingforce transmission system 17, and the bevel gear pairs 18 and 19. Then,the ball screw 20 rotates and the nut 21, that is, the nut housing 31which is engaged with the ball screw 20 rises up. Thus, the rotationforce of the motor 16 is converted into a reciprocative force, and thisforce is transmitted to the movable platen 13 through the double togglemechanism 30. In the initial state of operation the first clutch Cl isturned off, and the second clutch C2 is turned on, controlled by thecontrol apparatus 50. Accordingly, the rotation force of the ACservomotor 16 is transmitted through the second torque transmissionseries 172 having a small reduction ratio. As a result, the movableplaten 13 rises up quickly.

The position of the movable platen 13 is detected with high precision bythe position detection means 40. That is, the rack 41 fixed to themovable platen 13 moves together with the movement of the movable platen13, and the pinion 42a of the rotary encoder 42 is rotated by thatmovement transmitted through the gear 44. Thus the rotary encoder 42outputs pulses of a number in accordance with the stroke of the movableplaten 13, and these are counted by the control apparatus 50 to detectthe position thereof.

The operation of the double toggle mechanism 30 will be described ingreater detail. As the double toggle mechanism 30 is providedsymmetrically at the left and right side, only the operation of theright side portion is described.

When the nut housing 31 rises up by the rotation of the ball screw 20,the intermediate link 32 rotates in clockwise direction with thesupporting point 31a as its center together with the rising up of thenut housing 31. At the same time the L-shaped link 33 rotates inclockwise direction with the supporting point 33a as its center, wherebythe output link 34 pushes up the movable platen 13, rotating incounter-clockwise direction with the supporting point 34a as its center.As the angle θ shown in FIG. 3 gradually becomes smaller as the movableplaten 13, rises the double toggle mechanism 30 inherently generates atremendously large force in the neighborhood of a position where theangle θ becomes 0, that is, in the neighborhood of the uppermost deadpoint (refer to FIG. 5).

When the movable platen 13 rises up to the neighborhood of the moldclamping position and it is detected that the movable platen 13 has comeclose to a predetermined position by the position detection means 40,the control apparatus 50 controls the whole system in accordance withthe flowchart shown in FIG. 8.

At first the AC servomotor 16 is servolocked (step 60), and its rotationis stopped for a while. Thereafter, the brake 65 is driven to controlthe speed of the torque transmission system 17 (step 61), and theclutches are switched, that is, the second clutch C2 is turned off andthe first clutch Cl is turned on (step 62). Thereafter, the brake 51 isreleased (step 63), and the servolock of the AC servomotor 16 isreleased (step 64), and the motor 16 is restarted.

Thus, the rotation force of the AC servomotor 16 is transmitted throughthe first torque transmission series 171, and accordingly, the movableplaten 13 continues to rise up again at a slow speed and with a largedriving force, and the lower mold 15 and the upper mold 14 are moldclamped at a predetermined pressure. A plastic molding onto the leadframe is conducted in this mold clamping state. That is, the plunger 24provided at the lower mold 15 is driven by the driving apparatus (notshown), and thus the tablet 22 is injected into the mold. The pressureof the plunger 24 is held at the predetermined pressure for apredetermined time period, thereby to conduct a plastic molding.

When this plastic molding is completed, the movable platen 13 is loweredafter the plunger 24 and the tablet 22 are separated, and the plasticmolded lead frame is separated from the lower mold 15. Thereafter, theplastic molded lead frame is taken out. by an outloader to proceed tothe next process. Thereafter, the movable platen 13 rises up to thecleaning point, and the surface of the lower mold 15 is cleaned by acleaner and returns to the original position.

According to the present embodiment under such a construction, the meansfor obtaining a press pressure is constituted not by a conventional oilpressure system but by a motor-operated system, thereby enabling thepress pressure to be adjusted with high accuracy and a goodresponsibility, and a plastic molding is always conducted at a highaccuracy. Furthermore, the whole device includes no oil pressure systemfor the power unit is small in size and is low cost. The maintenancerequirement are lessened.

The torque transmission system is constituted by two different serieshaving different reduction ratios, thereby eliminating the necessity toprovide two AC servomotors one for high speed rotation and small torquecapacity for high speed forwarding, and one for low speed rotation andlarge torque capacity for mold clamping. The high speed forwarding andthe mold clamping can be conducted by one small capacity AC servomotor,resulting in a small sized and low cost motor.

A ball screw 20 and a nut 21 are used to constitute driving forceconversion mechanism of a high efficiency, and accordingly, there is apossibility that the movable platen 13 is lowered while the bothclutches C1 and C2 are off during the switching of the clutches. In thisembodiment, however, the torque transmission system 17 is braked by thebrake 65 before the clutch C2 is turned off, thereby preventing themovable platen 13 from lowering during in the switching period of thetorque transmission system 17. Accordingly, it is possible to conduct aposition detection accurately, and to conduct a plastic molding at highprecision by moving the movable platen 13 smoothly. Furthermore, thebrake 65 is provided at the input axis 17a, thereby enabling the use ofa brake having small brake torque capacity, whereby the device isfurther miniaturized and made less expensive.

Furthermore, in this embodiment under such construction the clutches C1and C2 are of normal open types which are off when a current is notflowing, and therefore, there might be an anxiety that the movableplaten 13 will lower when clutches C1 and C2 are turned off by such asthe stoppage of the power supply. However, a normal closed type brake 65in which a brake function is operated in a state where a current is notflowing can be used to secure the safety in an abnormal state.

In the illustrated embodiment the brake is provided at the input axis,but it can be provided at any place beyond the clutch, for example, atthe output axis. In this case, however, a brake having a large braketorque capacity by an amount corresponding to the reduction ratio isrequired as compared with the above embodiment.

In the illustrated embodiment the position of the platen 13 is detectedby using the rack 41 provided at the movable platen 13. This enablesless and precise position detection with no consideration with respectto sliding clutches or the link ratio of the toggle mechanism.Accordingly, it is possible to control the press pressure at the moldclamping securely, resulting in a plastic molding of high precision.

In the illustrated embodiment a rotary encoder is used as a positiondetection means, but a linear scale or a potentiometer can be used. Anincremental type encoder which outputs only pulses is used as a rotaryencoder, but an absolute type encoder which outputs an absolute positionby bit display can be used.

Furthermore, the mold clamp is conducted at the neighborhood of theuppermost dead point of the toggle mechanism, whereby the mold clampingcan be conducted at a very low speed. Accordingly, only a little shockarises at the mold clamping, whereby the molds are prevented from beingdamaged, and the lives of the molds are enhanced to a great extent. Itis prevented that positional deviations occur in the molds and goldwires of IC are cut off at the mold clamping, thereby enhancing thereliability of the semiconductor device.

FIG. 9 shows a second embodiment of the present invention. Thisembodiment requires less motor power during raising of the movableplaten, thereby may be miniaturized. This object is accomplished byproviding a spring for urging the movable platen upwards.

In FIG. 9, the reference numeral 25 designates a first coil springwhich, provided between the lower platen 11 and the movable platen 13,urges the movable platen 13 upwards. This coil spring 25 is provided insuch a manner that the tie bar 12 is inserted into the coil spring 25.The numeral 26 designates a second coil spring provided between the nuthousing 31 of the double toggle mechanism 30 and the lower platen 11,that is, between the driving force conversion mechanism in the motivepower transmission system (comprising the ball screw 20 and the nut 21)and the double toggle mechanism 30. It is sufficient for, the first coilspring 25 to have a short stroke but is required to have a large springconstant because it is provided beyond the double toggle mechanism 30 inthe motive force transmission system. The second coil spring 26 may havea small spring constant but is required to have a long stroke because itis provided at a former stage than the double toggle mechanism 30.

The operation of this embodiment is approximately the same as that ofthe first embodiment. However, the nut housing 31 and the movable platen13 are urged upwards by the second and first coil springs 26 and 25 atthe mold clamping, and therefore, the motor power at the rising of themovable platen 13 is reduced to a great extent. Accordingly, the motiveforce transmission system can be constructed by a transmission systemhaving two reduction ratios, with the larger one of them for obtaining adesired mold clamping pressure, and then the motor 16 itself isminiaturized.

Even if an abnormality arises in the electricity system or the drivingsystem, which prevents the movable platen 13, from completely rising thecoil springs 25 and 26 can operate as buffer elements, therebypreventing the movable platen 13 from lowering suddenly.

In the illustrated embodiment a first coil spring is provided betweenthe movable platen and the lower platen, and a second coil spring isprovided between the double toggle mechanism and the driving forceconversion mechanism, but only one of them may be provided.

Furthermore, the fundamental principle of the embodiment can be appliedto a plastic molding device having no double toggle mechanism.

A third embodiment of the present invention will be described for usewith the embodiment of FIG. 2. This embodiment is aiming at conducting acontrol of the press pressure easily and at high precision. The wholeconstruction thereof is the same as that shown in FIG. 2. The positiondetection means 40 in this embodiment is constituted so as to detectrelative positions of the upper mold 14 and the movable platen 13, andthe control apparatus 50 is constructed so as to control the driving ofthe AC servomotor 16 so that the relative position of the upper mold 14and the movable platen 13 becomes a desired one, with receiving theoutput signal of the rotary encoder 42 of the position detection means40.

The fundamental principle of the motor driving control in thisembodiment will be described.

It is well known that when a substance is pressed (or pulled) by apredetermined force, the following relationship is established betweenthe stress α which arises within the substance, the elastic distortionε, and the Young ratio of the substance E

    α=ε. E                                       (1)

In this embodiment it is intended to move the movable platen 13 up to aposition where an elastic distortion ε arises and the press pressure isequal to the target pressure with the use of the data of the relativeposition between the upper mold 14 and the movable platen 13 which isobtained by the position detection means, on an assumption that thepress pressure of the movable platen 13 corresponds to the stress α,,and the relative position between the upper mold 14 and the movableplaten 13, that is, the displacement of the movable platen 13corresponds to the elastic distortion ε. Hereupon, the Young ratio E canbe easily obtained by calculation or experiment from the wholeconstruction including the upper mold 14, the lower mold 15, and theplatens.

The device will be operated as follows:

The operation of this embodiment is basically the same as that of theabove embodiment, and only the operation concerning the control of thepress pressure will be described.

When the movable platen 13 rises up by the rotation of the AC servomotor16 and the position at which the upper and the lower molds 14 and 15contact with each other is detected, a rotation instruction is outputfrom the driving control apparatus 50 to the motor 16, which instructionis for instructing the motor 16 to rotate so as to rise up the movableplaten 13 by the amount of displacement x corresponding to a desiredpress pressure from the contact point. This displacement x can beobtained from the above described formula (1). When the position of themovable platen 13 is detected by the position detection means 40 and themovable platen 13 actually moves by the amount of displacement x, themotor 16 stops. In this state where the mold clamping is conducted at adesired press pressure a plastic molding is conducted. Herein, therelative position of the movable platen 13 and the upper mold 14 isdetected similarly as the above embodiment by counting the pulse signalsoutput from the rotary encoder 42 and detecting the position of themovable platen 13.

In this embodiment under such an operation, it is possible to controlthe press pressure easily and at a high preciseness by only detectingthe relative position of the movable platen 13 and the upper mold 14.

As the means for detecting the relative position in this embodiment anencoder which is usually provided at the AC servomotor can be used.

FIG. 10 shows a fourth embodiment of the present invention which has acharacteristic in the control of the press pressure. This fourthembodiment is constructed to operate in such a manner that the actualpress pressure is detected from the distortion which arises at apredetermined position of the device caused by the press, and thedriving force of the motor for driving the movable platen is controlledso that the difference between the detected press pressure and thetarget pressure may become zero.

In FIG. 10, the same reference numerals are used to designate the sameor corresponding elements as shown in FIG. 2. The reference numeral 45designates a strain gage for detecting the distortion of each of fourtie bars 12 so as to measure the actual press pressure. The referencenumeral 50 designates a control apparatus for controlling the drivingforce of the AC servomotor 16 so that the difference between the presspressure detected by the strain gage 40 and the target pressure maybecome zero.

FIG. 11 shows a construction of the control apparatus 50. The numeral 51designates an adder for conducting a subtraction between the measuredpress pressure and the target pressure. The numeral 52 designates aproportional integration control section having a proportional element52a and an integration element 52b. This is intended to conduct a PIcontrol by adding the result of the multiplication of the output of theadder 51 by a proportional constant Kp to the result obtained by furtherintegrating the above-mentioned result of multiplication. The numeral 53designates a limiter for conducting amplitude limiting of the signaloutput from the PI control section 52. The numeral 54 designates aconstant circuit for converting the output of the limiter 53 into acontrol signal to be input to the AC servomotor 16.

The device will be operated as follows:

Also in this embodiment the operation is such that the lower mold 15 ispushed towards the upper mold 14 by a predetermined press pressure afterthe movable platen 13 rises up by the rotation of the AC servomotor 16as in the above-described embodiments.

In this state the distortion of the tie bar 12, that is, the actualpress pressure is detected by the strain gage 45 provided at each tiebar 12. In the control apparatus 50 the difference between the detectedpress pressure and the target pressure is obtained by the adder 51, andthe output thereof is PI controlled by the PI control section 52. Thesignal obtained in this way is input to the torque limiting signal inputterminal of the AC servomotor 16 through the limiter 53 and the constantcircuit 54, and the motor 16 is driven under control so that the presspressure of the mold clamping by the lower mold 15 and the upper mold 14may become the target pressure. Besides, the plastic molding operationthereafter is the same as that of the conventional device.

Herein, the limiter 53 in the control apparatus 50 is intended tosuppress the control signal to within a narrow range in a state wherethe lower mold 15 does not contact with the upper mold 14, that is, nopress pressure is measured, whereby the output torque of the motor 16 islimited, for example, in a range of 200% of the predetermined ratedtorque. Accordingly, even if the movable platen 13 happens to stoprising up due to friction, the rotation torque of the motor 16 does notbecome a tremendously large value, thereby preventing the device frombeing destroyed.

In such an embodiment the press pressure which is actually applied isdetected by a strain gage 45, and the motor 16 is controlled so that thedetected press pressure may become a target pressure, whereby a desiredvalue of press pressure is obtained securely, and adjustment thereof canbe conducted easily and at high precision.

Besides, the portion at which the press pressure is to be detected fromdistortion is not limited to the tie bar described in the aboveembodiment, and any portion where a distortion arises such as someportion of the upper platen can be used.

A fifth embodiment of the present invention will be described withreference to FIG. 12. This embodiment is aiming at simplifying thestructure while avoiding oil leakage by using a belt transmission systemin the motive force transmission.

FIG. 12 shows an enlarged view of the motive force transmission systemand the double toggle mechanism. The motive force transmission system isconstituted in one series, and in such a construction it is possible torealize a device capable of obtaining a mold clamping force up to about80 tons with the use of an AC servomotor of 5 KW, or that capable ofobtaining a mold clamping force up to about 150 tons with the use of aservomotor of 7 KW, with a little variation depending on the reductionratio of the motive force transmission system.

In FIG. 12, the same numerals designate the same elements as those shownin FIG. 2. The numeral 70 designates a motive force transmission systemfor transmitting the rotation force of the AC servomotor 16 to the ballscrew 20 by a belt 75. In this motive force transmission system 70 thenumeral 71 designates an input axis connected to the output axis of themotor 16, the numeral 72 designates a small pulley fixed to the inputaxis 71, the numeral 73 designates an output axis provided at andintegral with the top end of the ball screw 20. The numeral 74designates a large pulley fixed to the output axis 73, and the numeral75 designates the belt hung in parallel over the small pulley 72 and thelarge pulley 74. The numerals 76a to 76ddesignate bearings forsupporting the input axis 71 and the output axis 73 rotatably.

The device will be operated as follows:

At first, when the operation start switch of the device is turned on,the AC servomotor 16 starts to rotate, and the input axis 71 and thesmall pulley 72 rotate at the same speed as that of the motor 16. Thisrotation force is transmitted to the large pulley 74 and the output axis73 through the belt 75, and the ball screw 20 is rotated at apredetermined rotation speed. The rotation speed of the ball screw 20 isdetermined by the reduction ratio determined by the radius ratio of thesmall pulley 72 and the large pulley 74. When this ball screw 20rotates, the nut engaged with the ball screw 20, that is, the nuthousing 31 rises up, and thus the rotation force of the motor 16 isconverted into a reciprocative force, and this reciprocative force istransmitted to the movable platen 13 through the double toggle mechanism30.

Besides, the operation of the sections other than the motive forcetransmission system is the same as those of the above describedembodiments.

In this embodiment, the motive force transmission system is constitutedby a belt mechanism, thereby simplifying the structure as compared withthe bigear transmission system in the above-described embodiments. Thenumber of elements is reduced thereby simplifying the construction ofthe device, the cost of the device is also reduced. There is no need toprovide oil for lubricating gears, thereby preventing anxiety such withregard to oil leakage, resulting in easy maintenance. Furthermore, thereare no gears, resulting in no noises of gear tooth sounds. This offerslow noise devices as compared with the above-described embodiments.

In the above-illustrated embodiment, the motive force transmissionsystem is constituted by only one stage, but this can be constituted ina two-stage construction such as comprising a first stage for high speedforwarding having a small reduction ratio and a second stage for moldclamping having a large reduction ratio, thereby enabling a large moldclamping force by a small motor.

In all the embodiments a ball screw which is rotated by a motor and anut which is engaged with the ball screw are used as a driving forceconversion mechanism, but this invention is not limited thereto and anyconstruction capable of converting the rotation force to a reciprocativeforce can be used.

In conducting a mold clamping in the prior art device the weight isusually applied to the center portion of the mold, and therefore themeeting face of the molds is curved convex towards upwards as shown inFIG. 13. (In this Figure a quite large distortion is shown forexplanation.) However, the surface of the metal mold is finished to aquite high precision of micron unit, and therefore if the molds aredistorted in the above-described manner, a dimensional deviation occursin the metal molds, thereby hurting the precision of the plasticmolding.

A sixth embodiment of the present invention aiming at solving thisproblem is shown in FIG. 14. The numeral 27 designates a driving rod 27for pressing the central portion of the movable platen 13 by the drivingforce of the AC servomotor 16. In this embodiment a driving forceconversion mechanism for converting the rotation force of the ACservomotor 16 into a reciprocative force is provided similarly as in theabove-described embodiments, and the driving rod 27 is provided to bedriven by the reciprocative force obtained by the driving forceconversion mechanism. The upper mold 14 and the lower mold 15 in thisembodiment are produced curved convex towards downwards, that is, convextowards the press side so that the meeting surface may become flat whilemold clamping. In other words, they are produced curved in a reversedirection by that amount they are curved in the mold clamping. Thequantity to be curved previously is determined by the press pressure andthe rigidness of the structural elements such as platens and upper andlower molds.

In this embodiment under such construction, the surfaces of the upperand lower molds 14, 15 are produced curved convex downwards, wherebyeven if the movable platen 13 and the upper platen 10 are curved by themold clamping, the meeting surface of the molds for conducting plasticmolding becomes flat as shown in FIG. 15. Accordingly, no dimensionaldeviation occurs caused by distortion in a plastic molding conducted inthis state, resulting in a high quality plastic molding.

In the above illustrated embodiment the upper and lower molds arepreviously designed to be curved, but the upper platen and the movableplaten may be previously designed to be curved by a predeterminedquantity with the same effect as described above.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A plastic molding device for plastic molding asemiconductor element on a lead frame comprising:an upper platen, alower platen, and a movable platen which are all interconnected by atleast one tie bar; an upper metal mold and a lower metal mold providedon the upper platen and the movable platen, respectively; a motor forapplying a driving force for driving the movable platen toward the upperplaten; means for detecting the relative position of the upper metalmold and the movable platen, said means for detecting outputting anabsolute value for avoiding the use of a counter; and a driving controlapparatus for driving the motor so that the movable platen reaches adesired position relative to the upper mold.
 2. A plastic molding devicefor plastic molding a semiconductor element on a lead framecomprising:an upper metal mold and a lower metal mold for plasticmolding which have a space therebetween containing a lead frame and asemiconductor element, each of said molds being produced in a matingcurved configuration; and means for pressing both metal molds togetherby applying a force to approximately the centers thereof, said curvedconfiguration of said molds being flattened when pressed together andreturning to said curved configuration after said molds are separated inorder to accommodate deformation of said molds during pressing.